1. Field of Invention
This invention relates to the field of compression springs formed of elastomers. More specifically, the invention is primarily directed to compression spring improvements for use in the suspension systems of automobiles.
Since the early 1970's, automobile designs have focused on lighter weight and lower body lines. These designs necessitate suspension system modifications in at least two respects. First, the lower body line necessitates reduced suspension travel. Second, the lighter weight necessitates changes to very soft spring rates. These changes, in our view, require further design improvements in the jounce and rebound bumpers of the suspension strut assembly. In spite of years of design of these light weight, low profile automobiles, their suspension systems still transmit undesirable noise, vibration and road shock to the passengers. As a result, a major automobile manufacturer requested the design of several alternatives to its current rebound bumper and expressed a need for a spring with a soft entry and a longer travel. Consequently, this invention is primarily related to the problems of the strut assemblies of automobiles.
2. Related Art
Rebound bumpers for strut assemblies are conventionally formed of urethane elastomers. Their purpose is merely to soften and dampen the rebound impact when the strut of the suspension assembly is extended its maximum length as the wheel assembly drops into a depression relative to the vehicle body. Presently, such bumpers comprise small urethane washers surrounding the piston of the strut and have such a limited thickness as to cushion the vehicle only upon the last 2 to 3 millimeters of rebound. Moreover, they have a constant, high spring rate that does little to attenuate the noise, vibration or impact forces.
Present jounce bumpers for strut assemblies have undesirable characteristics similar to those of the rebound bumpers. Some improvement in these bumpers has been obtained through the use of micro-cellular urethane. In addition, U.S. Pat. No. 4,962,916, which issued to Uniroyal Chemical Co., Inc. on an application of Palinkus, suggested jounce bumpers formed of thermosetting and thermoplastic polyurethanes in which the bumpers have varying wall thicknesses in order to provide for a variable spring rate. This patent also mentions copolyesters such as Hytrel.RTM.--but fails to address the compression set problem of such copolyesters or how this problem can be resolved with beneficial results. U.S. Pat. No. 4,235,427, which issued to Bialobrzeski, discloses the desirability of making springs of plastics such as acetal, polyethylene, polypropolene, tetrafluoroethylene, nylon and flexible PVC in which the characteristics of the spring is primarily determined by different radii of inner and outer convolutions whose resiliency is due to bending of the convolutions, not to the elastic deformation of the elastomeric material.
While unrelated to the present application, the prior art does include effective compression springs formed of copolymer elastomers such as Hytrel.RTM., an elastomer manufactured by E. I. du Pont de Nemoirs. Illustrative of this prior art are U.S. patents which issued to David G. Anderson. They include U.S. Pat. No. 4,198,037 entitled "Method of Making Polyester Elastomer Compression Spring and Resulting Product" and U.S. Pat. No. 4,566,678 entitled "Polymeric Apparatus and Method of Making the Same."
These patents of Mr. Anderson relate to compression springs that have a primary application in railroad car draft gears and other energy absorption devices. These applications involve a very high spring rate to absorb the impact forces generated by railroad cars weighing as much as 300,000 pounds. Such compression springs do not solve the problems faced by today's automobiles which demand a very soft ride in a light weight vehicle.